Aircraft gas turbine engines operate throughout an extensive operating envelope of altitudes, flight speeds and power settings. The widely varying operational conditions within the operating envelope impose differing, and sometimes conflicting demands on an engine. Therefore, engines are typically designed to exhibit peak performance within that portion of the operating envelope where the engine will be operated most frequently, with the consequence that engine performance is suboptimal at other conditions. It is also common practice to employ mechanical devices, such as variable pitch blades and vanes, as well as sophisticated control strategies to maximize the portion of the operating envelope over which engine performance will be at or near optimum. However these devices and strategies do not alter the fundamental thermodynamic cycle of the engine, and therefore cannot produce uniformly optimum performance throughout the entire operating envelope. Because such engines are based on a single thermodynamic cycle, they are categorized as single cycle engines. Most modern commercial engines are good examples of single cycle engines--they exhibit excellent fuel efficiency at cruise conditions (high altitude, high subsonic airspeed, moderate engine power) where most of the engine's operation occurs but may offer poorer fuel economy at other operating conditions where the engine spends relatively little of its operational life.
Variable cycle engines represent another approach to accommodating the variation of conditions within an engine's operating envelope. A variable cycle engine is operable in any of two or more modes, each of which corresponds to a distinctly different thermodynamic cycle. Each mode is customized to achieve the best possible performance within a portion of the operating envelope. For example, a carrier based military aircraft may require an engine with high specific thrust (thrust per unit airflow or thrust per unit frontal area) for limited duration bursts of power during takeoff, while also requiting good fuel economy to maximize the aircraft's combat radius. If a single cycle engine cannot satisfy both requirements, and neither requirement can be relaxed, a variable cycle engine may be able to satisfy the conflicting demands.
The cycle variability of a variable cycle engine is often achieved with systems of external doors which open or close to expose or block auxiliary air inlets or exhaust ports. Additionally or alternatively, internal doors or valves may open or close to reroute airflow within the engine. The operation of these systems must be carefully coordinated with the operation of other engine systems to ensure proper engine operation in each mode and smooth, trouble free transition between modes. Obviously, the disadvantages of a variable cycle engine include the complexity, weight and cost associated with these systems and their operation. The disadvantages are acceptable only if the engine satisfies the performance demands imposed upon it in all of its operational modes.
The present invention is concerned with a variable cycle engine in which internal valves redirect the flow of air within the engine to achieve multimodal operation. In one mode of operation, referred to as the low mode, the engine operates much like a conventional turbofan engine. Air drawn into the engine intake flows through a fan and then is split into coannular streams. One stream flows into a fan duct while the other is channeled into a core unit which includes a variable pitch guide vane array, a hybrid compression stage having a rotor blade array and a variable pitch stator vane array, and a core compressor. The streams are reunited at an internal mixing plane at the aft end of the fan duct and the reunited stream is discharged through an external exhaust nozzle. In this mode the engine produces sufficient specific thrust for cruise operation, exhibits low fuel consumption and operates at moderate internal temperatures. In another mode of operation, referred to as the high mode, airflow from the fan is blocked from entering the fan duct and instead is channeled to the engine's core unit and through the guide vane array. Upstream of the compressor, the air stream is split into two streams, one of which enters the compressor and the other of which flows through a compressor bypass port and into the fan duct. In the high mode the engine produces greater specific thrust than in the low mode, but does so at the expense of increased fuel consumption and elevated internal temperatures.
One shortcoming of the above described variable cycle engine is related to the difference in the quantity (mass flow per unit time) of air channeled into the core unit in each of the described modes. In the high mode, the vanes of the guide vane array are oriented at an open pitch angle to accommodate the large quantity of air channeled to the core unit. The vanes of the hybrid stage stator vane array are likewise oriented at an open pitch angle so that the incidence angle between the leading edges of the hybrid vanes and the local flow direction of the air stream is within a range of incidence angles that produces aerodynamically stable operation of the vanes. The open vane orientation also guarantees that the flow area of the hybrid vane array (i.e. the aggregate area of the intervane flow passages) is sufficient to accommodate the large quantity of air from the fan. When the engine is operated in the low mode, a significantly smaller quantity of air is channeled into the core unit at a correspondingly lower velocity. The guide vanes are oriented at a more closed pitch angle which is compatible with the reduced air velocity. The hybrid vanes are also oriented at a more closed pitch angle so that the incidence angle, which is affected by the reduction in velocity, remains within the range of aerodynamic stability. Unfortunately, the closed pitch angle also reduces the flow area of the hybrid vane array, constricting its flow capacity. The reduction in flow capacity exceeds the reduction in the quantity of air channeled to the core unit in the low mode (relative to the high mode). As a result, the air pressure downstream of the hybrid rotor blade array becomes elevated and makes the rotor blade array susceptible to an aerodynamic instability similar to the instability which closure of the hybrid vanes was intended to prevent.
This shortcoming can, of course, be overcome by reducing the difference in the quantity of air channeled to the core unit in each of the two modes. A reduction in the difference may make it possible to identify an orientation of the hybrid vanes that offers an acceptable incidence angle as well as adequate flow capacity in the low mode. However reducing the difference in the airflow quantities is tantamount to reducing the differences in the corresponding thermodynamic cycles and results in a reduction or elimination of the benefits of cycle variability. As discussed above, the complexity and cost of a variable cycle engine can only be justified if the potential benefits of cycle variability are realized.